1. Field of the Invention
The present invention relates to the method and means for reading a frequency modulated video signal stored in the form of successively positioned reflective and non-reflective regions on a plurality of information tracks carried by a video disc. More specifically, an optical system is employed for directing a reading beam to impinge upon the information track and for gathering the reflected signals modulated by the reflective and non-reflective regions of the information track. A frequency modulated electrical signal is recovered from the reflected light modulated signal. The recovered frequency modulated electrical signal is applied to a signal processing section wherein the recovered frequency modulated signal is prepared for application to a standard television receiver and/or monitor. The recovered light modulated signals are applied to a plurality of servo systems for providing control signals which are employed for keeping the lens at the optimum focus position with relation to the information bearing surface of the video disc and to maintain the focused light beam in a position such that the focused light spot impinges at the center of the information track.
2. Summary of the Invention
The present invention is directed to a video disc player operating to recover frequency modulated video signals from an information bearing surface of a video disc. The frequency modulated video information is stored in a plurality of concentric circles or a single spiral extending over an information bearing portion of the video disc surface. The frequency modulated video signal is represented by indicia arranged in track-like fashion on the information bearing surface portion of the video disc. The indicia comprise successively positioned reflective and non-reflective regions in the information track.
A laser is used as the source of a coherent light beam and an optical system is employed for focusing the light beam to a spot having a diameter approximately the same as the width of the indicia positioned in the information track. A microscopic objective lens is used for focusing the read beam to a spot and for gathering up the reflected light caused by the spot impinging upon successively positioned light reflective and light non-reflective regions. The use of the microscopically small indicia typically 0.5 microns in width and ranging between one micron and 1.5 microns in length taxes the resolving power of the lens to its fullest. In this relationship, the lens acts as a low pass filter. In the gathering of the reflected light and passing the reflected light through the lens when operating at the maximum resolution of the lens, the gathered light assumes a sinusoidal-shaped like modulated beam representing the frequency modulated video signals contained on the video disc member.
The outpt from the microscopic lens is applied to a signal recovery system wherein the reflected light beam is employed first as an information bearing light member and second as a control signal source for generating radial tracking errors and focus errors. The information bearing portion of the recovered frequency modulated video signal is applied to an FM processing system for preparation prior to transmission to a standard TV receiver and/or a TV monitor.
The control portion of the recovered frequency modulated video signal is applied to a plurality of servo subsystems for controlling the position of the reading beam on the center of the information track and for controlling the placing of the lens for gathering the maximum reflected light when the lens is positioned at its optimum focused position. A tangential servo subsystem is employed for determining the time base error introduced into the reading process due to the mechanics of the reading system. This time base error appears as a phase error in the recovered frequency modulated video signal.
The phase error is detected by comparing a selected portion of the recovered frequency modulated signal with an internally generated signal having the correct phase relationship with the predetermined portion of the recovered frequency modulated video signal. The predetermined relationship is established during the original recording on the video disc. In the preferred embodiment, the predetermined portion of the recovered frequency modulated video signal is the color burst signal. The internally generated reference frequency is the color subcarrier frequency. The color burst signal was originally recorded on the video disc under control of an identical color subcarrier frequency. The phase error detected in this comparison process is applied to a mirror moving in the tangential direction which adjusts the location at which the focused spot impinges upon the information track. The tangential mirror causes the spot to move along the information track either in the forward or reverse direction for providing an adjustment equal to the phase error detected in the comparison process. The tangential mirror in its broadest sense is a means for adjusting the time base of the signal read from the video disc member to adjust for time base errors injected by the mechanics of the reading system.
In an alternative form of the invention, the predetermined portion of the recovered frequency modulated video signal is added to the total recorded frequency modulated video signal at the time of recording and the same frequency is employed as the operating point for the highly controlled crystal oscillator used in the comparison process.
In the preferred embodiment when the video disc player is recovering frequency modulated video signals representing television pictures, the phase error comparison procedure is performed for each line of television information. The phase error is used for the entire line of television information for correcting the time base error for one full line of television information. In this manner, incremental changes are applied to correct for the time base error. These are constantly being recomputed for each line of television information.
A radial tracking servo subsystem is employed for maintaining radial tracking of the focused light spot on one information track. The radial tracking servo subsystem responds to the control signal portion of the recovered frequency modulated signal to develop an error signal indicating the offset from the preferred center of track position to the actual position. This tracking error is employed for controlling the movement of a radial tracking mirror to bring the light spot back into the center of track position.
The radial tracking servo subsystem operates in a closed loop mode of operation and in an open loop mode of operation. In the closed loop mode of operation, the differential tracking error derived from the recovered frequency modulated video signal is continuously applied through the radial tracking mirror to bring the focus spot back to the center of track position. In the open loop mode of operation, the differential tracking error is temporarily removed from controlling the operation of radial tracking mirror. In the open loop mode of operation, various combinations of signals take over control of the movement of the radial tracking mirror for directing the point of impingement of the focused spot from the preferred center of track position on a first track to a center of track position of an adjacent track. A first control pulse causes the tracking mirror to move the focused spot of light from the center of track position on a first track and move towards a next adjacent track. This first control pulse terminates at a point prior to the focused spot reaching the center of track position in the next adjacent track. After the termination of the first control pulse, a second control pulse is applied to the radial tracking mirror to compensate for the additional energy added to the tracking mirror by the first control pulse. The second control pulse is employed for bringing the focused spot into the preferred center of track focus position as soon as possible. The second control pulse is also employed for preventing oscillation of the read spot about the second information track. A residual portion of the differential tracking error is also applied to the radial tracking mirror at a point calculated to assist the second control pulse in bringing the focused spot to rest at the center of track focus position in the next adjacent track.
A stop motion subsystem is employed as a means for generating a plurality of control signals for application to the tracking servo subsystem to achieve the movement of a focused spot tracking the center of a first information track to a separate and spaced location in which the spot begins tracking the center of the next adjacent information track. The stop motion subsystem performs its function by detecting a predetermined signal recovered from the frequency modulated video signal which indicates the proper position within the recovered frequency modulated video signal at which time the jumping operation should be initiated. This detection function is achieved, in part, by internally generating a gating circuit indicating that portion of the recovered frequency modulated video signal within which the predetermined signal should be located.
In response to the predetermined signal, which is called in the referred embodiment a white flag, the stop motion servo subsystem generates a first control signal for application to the tracking servo subsystem for temporarily interrupting the application of the differential tracking error to the radial tracking mirrors. The stop motion subsystem generates a second control signal for application to the radial tracking mirrors for causing the radial tracking mirrors to leave the center of tracking position on a first information track and jump to an adjacent information track. The stop motion subsystem terminates the second control signal prior to the focus spot reaching the center of focus position on the next adjacent information track.
In the preferred embodiment, a third control signal is generated by the stop motion subsystem at a time spaced from the termination of the second control pulse. The third control pulse is applied directly to the radial tracking mirrors for compensating for the effects on the radial tracking mirror which were added to the radial tracking mirror by the second control pulse. While the second control pulse is necessary to have the reading beam more from a first information track to an adjacent information track, the spaces involved are so small that the jumping operation cannot always reliably be achieved using the second control signal alone. In a preferred embodiment having an improved reliable mode of operation, the third control signal is employed for compensating for the effects of the second control jump pulse on the radial tracking mirror at a point in time when it is assured that the focus spot has, in fact, left the first information track and has yet to be properly positioned in the center of the next adjacent information track. A further embodiment gates the differential error signal through to the radial tracking mirror at a time calculated for the gated portion of the differential tracking error to assist the compensation pulse in bringing the focus spot under control upon the center of track position of the next adjacent information track.
The video disc player employs a spindle servo subsystem for rotating the video disc member positioned upon the spindle at a predetermined frequency. In the preferred embodiment the predetermined frequency is 1799.1 revolutions per minute. In one revolution of the video disc, a complete frame of television information is read from the video disc, processed in electronic portion of the video disc player and applied to a standard television receiver and/or television monitor in a form acceptable to each such unit, respectively. Both the television receiver and the television monitor handle the signals applied thereto by standard internal circuitry and display the color, or black and white signal, on the receiver or monitor.
The spindle servo subsystem achieves the accurate speed of rotation by comparing the actual speed of rotation with a motor reference frequency. The motor reference frequency is derived from the color subcarrier frequency which is also used to correct for time base errors as described hereinbefore. By utilizing the color subcarrier frequency as the source of the motor reference signal, the spindle motor itself removes all fixed time base errors which arise from a mismatching of the recording speed with the playback speed. The recording speed is also controlled by the color frequency subcarrier frequency. The use of a single highly controlled frequency in both the recording mode and the reading back mode removes the major portion of time base error. While the color subcarrier frequency is shown as the preferred source in generating the motor reference frequency, other highly controlled frequency signals can be used in controlling the writing and reading of frequency modulated video signal on the video disc.
A carriage servo subsystem operates in a close loop mode of operation to move the carriage assembly to the specific location under the direction of a plurality of current generators. The carriage servo subsystem controls the relative positioning of the video disc and the optical system used to form the read beam.
A plurality of individual current sources are individually activated by command signals from the function generator for directing the movement of the carriage servo.
A first command signal can direct the carriage servo subsystem to move the carriage assembly to a predetermined location such that the read beam intersects a predetermined portion of the information bearing surface of the video disc member. A second current source provides a continuous bias current for directing the carriage assembly to move in a fixed direction at a predetermined speed. A further current source generates a current signal of fixed magnitude and variable length for moving the carriage assembly at a high rate of speed in a predetermined direction.
A carriage tachometer current generating means is mechanically connected to the carriage motor and is employed for generating a current indicating the instantaneous position and speed of the carriage motor. The current from the carriage tachometer is compared with the sum of the currents being generated in the current sources in a summation circuit. The summation circuit detects the difference between the current sources and the carriage tachometer and applies a different signal to a power amplifier for moving the carriage assembly under the control of the current generators.